This invention relates to tetrafluroethylene polymers and in particular to manufacture of articles comprising microporous, that is, low density, sintered polytetrafluoroethylene. The invention provides a simple process for forming elongated articles of low density, sintered tetrafluoroethylene polymers having a relatively low tensile strength, for example, in the form of electrical insulation disposed about a conductive core, to produce a microporous polytetrafluoroethylene article, such as polytetrafluoroethylene insulated cable characterized by improved strippability of the insulation.
"Porous" sold dielectric materials have found favor as insulation for electric cables used in communications and in the computer industry because the reduction in dielectric constant produced by the incorporation of air in a solid dielectric greatly improves the electrical characteristics of cable made with such insulation. Moreover, the porosity introduced into the dielectric reduces the weight of the cable and concommitently its cost. The latter is a particularly important consideration in the case of relatively expensive dielectric materials such as polytetrafluoroethylene. The lower dielectric constant of "porous" dielectrics also reduces the overall size of insulation required to achieve a given characteristic cable impedence.
Low density, sintered polytetrafluoroethylene which is microprocess has heretofore been described in Japanese Patent Publication No. 13,560/67 and U.S. Pat. 3,953,566, for example. It has been made by a process of stretching and consequently is also called "stretched" or "expanded" polytetrafluoroethylene. As described in the literature, the product is made by extruding a paste of extrusion grade unsintered polytetrafluoroethylene admixed with an extrusion aid. The extrusion aid is removed from the extrudate after extrusion, and the product is then stretched in one or more directions and sintered while holding it in stretched condition. Stretching causes the polymer in effect to decrease in density without significant decrease in dimensions transverse to the direction of stretching. Stretched polymer can be produced which is thus increased in volume by several hundred percent of its original volume, introducing micropores such that the finished product has "porosity" with the pore volume often accounting for a major portion of the total product volume. Increases in tensile strength of the product in the direction of stretch are achieved, which become substantial after sintering while holding it in stretched condition.
The extrusion process itself is conventional as practiced in the industry and called "paste extrusion". Typically extrusion aid is admixed with the extrusion grade unsintered polytetrafluorethylene powder in a proportion of 10 to 35% by volume. Preferably, the extrusion aid is V.M. & P. naptha and is employed in 18% by volume. The resultant paste, which is formed by tumbling or a similar procedure, is pressed into a preform shaped to be received in the barrel of a ram extruder. In the extruder, the preform is forced through a die with substantial reduction in cross-section. Sheeting dies and calendaring steps are commonly employed in conjunction with extrusion of the paste in order to obtain flat stock and in order to promote biaxial orientation of the fibers which are produced when unsintered polytetrafluoroethylene is extruded. Paste extrusion is also employed in manufacture of polytetrafluoroethylene insulated wire.
In producing low density, sintered polytetrafluoroethylene, the extrusion aid is removed from the extruded product by heating at mild temperatures to drive off the extrusion aid or by solvent leaching.
The product is then stretched in at least one direction, for example, by passing it in tape form over a roll travelling at a given speed and then to a capstan travelling at a faster speed such that the product is placed under tension and stretches between the roll and capstan. The stretching in length of the tape which takes place between the roll and capstan is not accompanied by any significant reduction in cross-section of the tape, thus, in effect increasing the volume and lowering the density of the material by stretching the spaces between the ibrous particles of polytetrafluoroethylene as well as elongating the fibers themselves.
The resulting low density polytetrafluoroethylene is soft and upon heating without restraint shrinks losing the microporosity achieved by stretching. It has been found, however, that if low density, unsintered polytetrafluoroethylene is heated to sintering temperatures while restrained in stretched condition the porosity becomes set and is retained after cooling with a significant increase in tensile strength of the product in the direction of the stretch over the tensile strength of sintered product which has not been stretched.
In the prior art processes, the resulting low density sintered polytetrafluoroethylene has a relatively high longitudinal tensile strength and longitudinal matrix tensile strength which is undesirable for various applications particularly, for insulation for wire or cable. Thus, with insulated wire or cable, adequate tensile strength is provided by the wire within the insulation, and it is desirable to have a low longitudinal tensile strength for strippability reasons. Thus, when terminating a wire or cable, a cut is made in the insulation, but such cut does not extend to the outer surface of the wire in order to avoid damage to the wire. Thereafter, the portion of the insulation to be removed is removed to bare the end of the wire, the insulation parting at the cut. The ease with which such portion of the insulation can be removed, or stripped, depends upon, and is an inverse function of the tensile strength of the insulation. Similar considerations apply to other articles of sintered, low density polytetrafluoroethylene which do not require a high tensile strength.
Low density, sintered polytetrafluoroethylene made as described above has been used in the manufacture of insulated cable by making low density sintered tape as described above, and then winding the tape helically with overlap about a conductor. Typically several layers of helical windings are positioned over the conductor. While the final insulated cable has the advantages of a "porous" dielectric in terms of significant weight reduction, size reduction and the like, the product has several drawbacks. The surface, naturally, is rough because of overlap in applying the helical servings of polytetrafluoroethylene tape. Consequently, it cannot be color coded or otherwise marked as well as might be desired. The compression on the inner layers of tape caused by the tension imposed during winding the outer layers results in partial collapse of the inner layers (increase in density) which makes it difficult to control the impedance of the cable. Also the cable does not strip cleanly. Crossed fibers from the biaxially oriented wrapped tapes resist clean breakage. Where the tapes adhere to each other, the dielectric is discontinuous at tape boundaries.